Carefully twist off the mushroom stalks. Brush the caps with oil. Mince the stalks and onion togethe, and place in a small bowl. Add the butter and MICROWAVE ON HIGH for 4 minutes, until soft. Stir in the ham, breadcrumbs and tomato ketchup. Spread over the flat side of the mushroom caps and arrange in a large shallow dish. Cover and MICROWAVE ON HIGH for 4-5 minutes until the mushrooms are tender, giving the dish a half-turn after 2 minutes. Garnish with parsley.Serves 4